Method and apparatus for detecting superimposed sheets of paper

ABSTRACT

A method and apparatus for detecting superimposed sheets of paper in a sheet fed printing press utilizes cooperating upper and lower transmitter and receiver pairs that are positioned above and below the sheet transport path. Voltages that are representative of the positions of the upper and lower surfaces of the sheet are compared to nominal values. If the actual values significantly exeed the nominal values, a superimposed sheet condition exists and appropriate corrective action can be taken.

FIELD OF THE INVENTION

The present invention is directed generally to a method and apparatusfor detecting superimposed sheets of paper. More particularly, thepresent invention is directed to a method and apparatus for detectingsuperimposed sheets of paper in a sheet transport path. Mostspecifically, the present invention is directed to a method andapparatus for detecting superimposed sheets of paper in a sheettransport path of a sheet fed rotary printing press. The sheets of paperare passed along a sheet feeding table and move between spaced lighttransmitters which may be laser diodes. The light reflected off thesurface of the sheet or sheets has an angle with respect to a receiverwhich depends on the distance of the sheet or sheets from the lighttransmitter. By analyzing output voltages from receivers placed bothabove and below the sheet or sheets, a value corresponding to thethickness of the sheet or sheets can be ascertained. A thickness valveoutside of a certain range is an indication of the superimposition oftwo or more sheets.

DESCRIPTION OF THE PRIOR ART

In sheet fed printing machines, a plurality of individual sheets aretypically fed along a path of travel through various printing, drying,arranging and other handling and processing stations or areas. It isimperative that the sheets be positioned one after each other and thatthey not be overlapped or superimposed. If two or more sheets do becomeoverlapped or superimposed the subsequent operation performed on theseveral sheets will quite probably not be performed correctly. Thus itis important that any overlapping or superimposition of sheets bedetected and corrected.

One device which is useable to control overlapped sheets is disclosed inU.S. Pat. No. 4,397,460. In this device, the sheets on a sheet transportway are checked to see whether or not they are overlapped orsuperimposed. This is accomplished by providing two sequentiallyarranged sensor sensor devices positioned above the sheet transport wayand two sequentially arranged sensor devices positioned beneath thesheet transport way or path. Each of these four sensor devices consistsof a light emitting diode and of a phototransistor sensor which receivesthe light emitted from the diode and reflected from a surface of thesheet.

In the device for the control of overlapping sheets as set forth in U.S.Pat. No. 4,397,460 it is necessary that the sheets to be measured beguided essentially absolutely parallel to the transport way or path inthe measuring range of the sequentially arranged sensor devices. This isaccomplished by providing vacuum devices that feed the sheets against asurface whose location and position has previously been determined. Inaddition, in the prior art sheet control device of this patent, thedegree of light reflection is measured and is thus dependant on theintensity of the reflection. This means that an accurate measurement canonly be accomplished on a surface having a uniform degree or amount ofreflection. This means that a measurement within a printed surface thatmay have been printed with several colors having differing intensitiesof reflection is not possible.

It will thus be apparent that a need exists for a method and apparatusfor detecting superimposed sheets of paper which overcomes thelimitations of the prior art. The assembly for detecting superimposedsheets in accordance with the present invention provides such a methodand apparatus and is a substantial improvement over prior art devices.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method andapparatus for detecting superimposed sheets.

Another object of the present invention is to provide a method andapparatus for detecting superimposed sheets on a sheet transport path.

A further object of the present invention is to provide a method andapparatus for detecting superimposed sheets on a sheet transport path ofa sheet fed rotary printing press.

Yet another object of the present invention is to provide a method andapparatus for detecting superimposed sheets which is independent fromthe degree of reflection of the surface of the sheet.

Still a further object of the present invention is to provide a methodand apparatus for detecting superimposed sheets which compensates fordecreasing radiant intensity.

As will be set forth in detail in the description of the preferredembodiment which is presented subsequently, the method and apparatus fordetecting superimposed sheets in accordance with the present inventionutilizes transmitters and cooperating spaced receivers which arepositioned both above and beneath the sheet transport way. Eachtransmitter is preferably a laser diode which directs a light against asurface of the sheet. The receiver is preferably a uniaxial positionsensitive diode operator which supplies an analog signal dependant onthe intensity distribution of light along its longitudinal axis. Theoutput of the position sensitive diode operator is useable both tocontrol the intensity of the laser diode transmitter and to measure thedistance from the sheet which is reflecting the signal from the laserdiode transmitter. By providing similar receivers that measure thedistances to both the top and bottom surfaces of the sheet, a valuerepresentative of the thickness of the sheet can be obtained. Thisactual value is compared with a previously obtained nominal value. Toogreat a difference in the two values will result in the generation of asignal indicating a superimposed sheet situation.

A principal advantage of the present invention lies in its ability to bepositioned at any point or points along the path of sheet transport. Theoverlapping or superimposition of the sheets can be sensed generallyindependently of the distance from the sheet surfaces to the sensor.This makes the present invention much more versatile than the prior artdevices.

The sheet superimposition valuation accomplished by the subjectinvention is done using a simple and clear connection between thereceiving unit and an assembly which stores previously ascertainednominal values In addition, the present invention provides a devicewhich compensates for decreasing radiant intensity.

It will be seen that the method and apparatus for detecting superimposedsheets in accordance with the present invention overcomes the limitationof the prior art. It is accordingly a significant advance in the art.

BRIEF DESCRIPTION OF THE DRAWINGS

While the novel features of the method and apparatus for detectingsuperimposed sheets of paper in accordance with the present inventionare set forth with particularity in the appended claims, a full andcomplete understanding of the invention may be had by referring to thedetailed description of preferred embodiments which is presentedsubsequently, and as shown in the accompanying drawings in which:

FIG. 1 is a schematic side elevation view of a sheet feeding table of arotary printing press utilizing the method and apparatus for detectingsuperimposed sheets of paper in accordance with the present invention;

FIG. 2 is a schematic depiction of the apparatus of the presentinvention and showing a first preferred embodiment of the path of thelight rays;

FIG. 3 is a schematic depiction of the apparatus and showing a secondpreferred embodiment of the path of the light rays; and

FIG. 4 is a schematic depiction of the evaluation connection of thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring initially to FIG. 1, there may be seen a sheet fed rotaryprinting press utilizing the method and apparatus for detectingsuperimposed sheets of paper in accordance with the present invention.The sheet fed rotary printing press shown in FIG. 1 has, among otherfeatures, a stop drum 1 and a sheet feeding table 2 with a sheettransport way 3. In a section of the sheet transport way 3, there isprovided a controlling device 4 for the control of overlapped sheets.The controlling device 4 has among other through a first transmitter 6and receiver 7, arranged above the transport way 3 in a housing 23, aswell as a second transmitter 8 and receiver 9, arranged in a housing 24under the transport way 3. These transmitters 6 and 8, and receivers 7and 9 may be seen most clearly in FIGS. 2 and 3. The housings 23 and 24are not shown in FIGS. 2 and 3 but are shown in FIG. 1.

The control device 4, and especially the receivers 7 and 9 thereof, areconnected with each other by means of an evaluation connection 11, whichlinks, at its output end, two analog voltages V₇ and V₉ with each otherand compares them with a previously stored nominal value. Each of thetwo voltage signals V₇ and V₉ is proportional to the distance of atarget 12, such as a sheet, from the respective receivers 7 or 9. Theevaluation connection 11 is shown in detail in FIG. 4 and will bediscussed in detail subsequently.

The arrangement of the transmitters 6 and 8 in the first embodiment isexecuted as may be seen in FIG. 2, so that both transmitters 6 and 8 arefacing each other at a double working distance× which may be, forexample 2×40 mm =80 mm, and the sheet 12 to be scanned is passing at anangle α of approximately 40° to 130° through a light ray 13 or 14,generated by a transmitter 6 or 8. As will be discussed shortly, thetransmitters 6 and 8 are preferably laser diodes that thus generate acoherent laser beam or light ray 13 or 14 through which the sheet 12passes.

The controlling device 4 in accordance with the present inventionoperates according to the optical principle of the triangulation method.This method is based on the fact that an object, such as a gauge spot16, which is positioned in front of an objective lens 17 or 18, yieldsbehind the objective lens a reversed real image. When the object 16 ispositioned a certain distance from the optical axis 19 or 21, it issharply projected in the image distance. In case the position of theobject 16 is changing, the position of the image is changing verticallyand parallely in relation to the optical axis 19 or 21.

Each transmitter 6 and 8 is preferably a pulsed laser diode. The laserdiode 6 or 8 projects a corresponding light spot or gauge spot 16 on thesheet 12 to be measured. The paper thickness of sheet 12 can be measuredwithin a working range of y such as +/- 10 mm. During the measurementoperation, the sheet 12 can move into any position. A convex lens 5 or10 prevents an overly high expansion of the light ray 13 or 14. With ameasuring range y of about +/-10 mm around the center distance of 40 mm,the measurement is effected in the convergent path of the rays of thelaser. For this reason, there is obtained a light spot, diminishing withincreasing measuring distance. When a focal distance of 80 mm isexceeded, the laser ray diverges and so experiences in a distance of 74cm such a big expansion, that there is no longer a danger for the humaneye, because of the decreasing illuminating power resulting from this.The NOHO-valve (Normal Optical Hazard Distance) of the laser transmitter6 or 8 is thus 74 cm.

The laser ray 13 or 14 passes parallely in relation to the transmitteraxis, so that during a a changing measuring distance to the sheet 12,the light spot 16 always has the same position on the sheet 12. Theoptical receiving unit 7 or 9 is arranged at a suitable angle β of about40° to 130° in relation to the transmitting ray 13 or 14, which projectsthe light spot 16 on the photoreceiver 7 or 9. When the sheet 12 movesvertically along the transmitting ray 13 or 14, the location of thereflected light on the photo-receiver 7 or 9 also moves, because of thisangle β. The arrangement angles α and β should be chosen so, that α isnot equal to β.

Each receiver 7 and 9 is a uniaxial PSD or position sensitivediode-operator. A PSD-operator is a photo-receiver 7 or 9, whose output,in dependence on the light intensity distribution along its longitudinalaxis 31 or 32, is analog signals I₁ and I₂ or I₃ and I₄. These outputanalog signals depend on the position of the reflected ray. The functionbecomes clear by comparison with a potentiometer. The proportion of thetwo output currents I₁ and I₂ or I₃ and I₄ is reversed proportional tothe relation of the paths a and b, which are determined by the positionof the light spot on the receivers 7 or 9. The sum Σ of the currents I₁and I₂ is equivalent to the light intensity Io of the light spot. Thephotoreceivers 7 and 9 each supplies a current I_(o) depending on theemitted illuminating power. This so-called monitor current I_(o)regulates the illuminating power of the laser diode 6 or 8 to 2 mW. Thiscompensates for any drift of the laser and thus there is guaranteed aconstant light emission in the working temperature range of 0°-50° C. ofthe apparatus in accordance with the present invention.

The sum Σ of the currents I₁ and I₂ is also a measure for the luminancefactor of the measured object 16. This value also intervenes in theluminous flux regulation of the laser diode or transmitter 6 or 8 andassures that the receiver 7 or 9 is always exposed with a sameintensity, independent of the object's color and surface. By this, thereis avoided an under-or overexposure of the photo-receiver 7 or 9 througha wide range.

The resolution is determined substantially by the noise and thetemperature coefficient of the PSD-receiver 7 or 9, and by imagedistortions of the optics 17 and 18. Since the light spot 16 and itsprojection have a finite extent, there occurs, depending on the colorand the kind of surface of the target 12, a non-homogeneous projectionof the light spot 16. When passing from a black to a white area, thelight spot 16 will have a higher intensity in the white area than in theblack area. So, it is not homogeneous. Depending on the roughness gradeof the surface of the sheet I2, there can also occur shadows in theprojection of the light spot 16. In this case as well, the projection ofthe light spot 16 at the receiver 7 or 9 is not homogeneous. As thesurface center of the energy distribution on the photo-receiver 7 or 9is determined by the evaluation connection 11, there occurs in the aboveexamples in consequences of an uneven projection of the light spot 16,an influence of the measured value. The linearity of the evaluationconnection 11 is obtained by a characteristic curve of the PSD-operatorand by the optical distortion of the projection of the light spot. Forthis reason, the same is corrected by a non-linear amplifier 41 or 42.These non-linear amplifiers are depicted schematically in FIG. 4.

Both photo-receivers 7 and 9 measure simultaneously the distance to thesheet 12 and both supply an analog voltage V₇ or V₉ which isproportional to the distance from the gauge spot 16 to thephoto-receiver 7 or 9. The two analog signals V₇ and V₉ are digitized bymeans of an input card 26 and supplied to a computer unit 27, as isdepicted schematically in FIG. 4. It will be understood that the inputcard 26 is representative of any known device which will covert ananalog signal to a digital signal that is understandable by the computerunit 27. The computer unit 27 determines from the analog voltages V₇ andV₉ the paper thickness and compares the value with a nominal paperthickness value that was stored during a prior "Learn"-cycle. During the"Learn"-cycle, a single sheet 12 is led over the sheet transport way 3and the paper thickness is measured by means of the controlling device4. This paper thickness value is then stored as the nominal value in thememory of the computer 27. It is, of course, also possible to put thenominal value into the computer 27 by means of a key board. In case themeasured paper thickness value diverges from the nominal measured value,beyond previously established limits, the computer unit then signalsthat overlapped sheets are existing.

Referring again to FIG. 2, it will be seen that the arrangement of thelaser transmitters 6 and 8 is executed so that the two laser rays 13 and14 have an angle of incidence α of 90° on the paper 12. By this, thelaser-overlapped sheet control is able to control a wide range of paperthicknesses such as from 40 g to 500 g. The advantage of thelaser-overlapped sheet control lies in that very thin or transparentsheets as well as very thick paper or cardboard can be controlled at thefeeder, previous to the pull lay, for overlapped sheets. In case thereare utilized, for example, advance sheets having the same paper weightas those to be utilized for the current order, the measuring method ofthe present invention does not make any difference for printed or blankpaper. External influences such as temperature, air humidity, daylight,ambient light or shocks also do not have any influence on the measuringresult.

Turning now to FIG. 3, there may be seen a second preferred embodimentof a method and apparatus for detecting superimposed sheets of paper inaccordance with the present invention. In this second embodiment shownin FIG. 3, each of the transmitters 6 and 8 is positioned in such a waywith respect to the sheet surface 12 that the laser rays 13 and 14impact the sheet surface at an angle α o of approximately 45°. In thiscase, the light spot 16 adopts an elliptic form with a varyinglyenlarged surface area such as approximately 0.8×0.4 mm at 30 mmdistance; 0.4×0.2 mm at 50 mm distance; or 0.6×0.3 mm at 40 mm distance;and with a similarly varying illuminating intensity. The optical axis 19or 21 of the reflected ray is arranged in this case rectangularly inrelation to the sheet surface or the sheet transport way 3.

As has been discussed briefly, FIG. 4 shows the evaluation connection 11for the currents I₁, I₂, I₃ and I₄. The currents I₁ and I₂ from thefirst receiver 7 are led to a first adding evaluating operator 33 andthe currents I₃ and I₄ from the second receiver 9 are led to a secondadding evaluating operator 34. Simultaneously, the currents I₁ and I₂are led to a first difference evaluating operator 36 and the currents I₃and I₄ are led to a second difference evaluating operator 37. In each ofthe first or second difference evaluating operators 36 and 37, thecurrent I₂ is subtracted from the current I₁ or the current I₄ issubtracted from the current I₃. ##EQU1##

The current values from the first adding evaluating operator 33 and thefirst difference evaluating operator are then fed to a first divideroperator 38. In a similar manner, the current values from the secondadding evaluating operator 34 and the second difference evaluatingoperator 37 are fed to a second divider operator 39. Each of the firstand second divider operators 38 and 39 divides the sum of the currentsI₁ +I₂ or I₃ +I₄ by the differences of the currents I₁ -I₂ or I₃ -I₄.Each of the resulting values is then led to the first or secondcooperating non-linear amplifier 41 or 42. From there, they come as thecomparison voltages V₇ and V₉ to the input card 26 of the computer 27.At the computer 27 these actual voltages V₇ and V₉, which are useable toascertain the actual thickness of the sheet 12, are compared to thenominal value of the paper thickness that has been previously enteredinto the computer 27. If the actual and the nominal thickness valuesdiverge considerably from each other so that this divergence exceeds apreviously selected limit, such as, for example if the actual value ismore than 1.8 times as great as the nominal value, a signal isgenerated. This signal may be generated using a suitable output card 28which gives an indication that an overlapping sheet condition exists.This signal can be used to accomplish the immediate stop of the sheettransport and can also generate suitable optical and/or acousticalsignals. As a result of the stopping of sheet transport and thegeneration of suitable signals, the press operator can rectify theoverlapping or superimposed sheet condition and can take the appropriateaction to prevent its recurrence.

While preferred embodiments of a method and apparatus for detectingsuperimposed sheets of paper in accordance with the present inventionhave been set forth fully and completely hereinabove, it will beapparent to one of skill in the art that a number of changes in, forexample, the sizes of the sheets being fed, the type of sheet feedingtable, the size of the stop drum and the like can be made withoutdeparting from the true spirit and scope of the subject invention whichis accordingly to be limited only by the following claims.

What is claimed is:
 1. A method for detecting superimposed sheets alonga sheet transport way, comprising:positioning a first light transmitteron a first side of said sheet transport way and focusing first lightrays from said first transmitter onto a light spot on first surfaces ofsheets positioned on said transport way; positioning a second lighttransmitter on a second side of said sheet transport way and focusingsecond light rays from said second transmitter onto a light spot onsecond surfaces of sheets positioned on said transport way; receiving ata first position-sensitive light receiver located on said first side ofsaid sheet transport way first light rays reflected from the light spoton first surfaces of sheets positioned on said transport way, theposition of incidence of said reflected light rays on said firstreceiver being dependent upon the distance between said light spot onsaid first surfaces and said first receiver; generating at said firstreceiver first and second analog signals whose values are proportionalto the position of incidence of the light rays reflected from said firstlight spot on said first receiver; summing said first and second signalsto produce first sum signals to measure and control the luminance ofsaid first light transmitter; receiving at a second position-sensitivelight receiver located on said second side of said sheet transport waysecond light rays reflected from the light spot on second surfaces ofsheets positioned on said transport way, the position of incidence ofsaid reflected light rays on said second receiver being dependent uponthe distance between said light spot on said second surfaces and saidsecond receiver; generating at said second receiver third and fourthanalog signals whose values are proportional to the position ofincidence of the light rays reflected from said second light spot onsaid second receiver; summing said third and fourth signals to producesecond sum signals to measure and control the luminance of said secondlight transmitter; subtracting said first and second signals to producefirst difference signals; subtracting said third and fourth signals toproduce second difference signals; dividing said first sum signals bysaid first difference signals to produce first evaluation signals forthe first surface of said sheets; compensating said first evaluationsignals for inhomogeneities in said first surfaces of said sheets toproduce a first surface analog position signal; dividing said second sumsignals by said second difference signals to produce second evaluationsignals for the second surface of said sheets; compensating said secondevaluation signals for inhomogeneities in said second surface of saidsheets to produce a second surface analog position signal; comparingsaid first surface position signal to said second surface positionsignal to obtain a sheet thickness value signal; and comparing saidsheet thickness value to a predetermined nominal thickness value todetect the presence of superimposed sheets.
 2. Apparatus for detectingsuperimposed sheets along a sheet transport way, comprising:first andsecond light transmitter producing first and second light rays,respectively, said first transmitter being located on a first side of asheet transport way and said second transmitter being located on asecond side of said sheet transport way; first and second focusing meansfor focusing said first and second rays, respectively, onto light spotson corresponding first and second surfaces of sheets on said sheettransport way; first and second position-sensitive light receiverslocated on said first and second sides of said transport ways,respectively, to receive light rays reflected from the light spots oncorresponding first and second surfaces of sheets positioned on saidtransport way, the position of incidence of reflected light rays on eachsaid receiver being dependent upon the distance between said receiversand their correspondence light spots on the sheet surfaces, saidreceives each generating first and second analog output surfaces whosevalues are proportional to the position of incidence of the light raysreflected from corresponding light spots on said corresponding first andsecond surfaces of sheets on said transport way; first and secondsumming means for receiving and summing said output signals of each ofsaid first and second receivers, respectively, to produce first andsecond sum signals for controlling the luminance of said first andsecond light transmitters, respectively; first and second subtractionmeans for receiving and subtracting the first and second output signalsof said first and second receivers, respectively, to produce first andsecond difference signals; first divider means dividing said first sumsignals by said first difference signals to produce first surfaceposition signals for the first surface of sheets on said transport way;second divider means dividing said second sum signal by said seconddifference signal to produce second surface position signals for thesecond surface of sheets on said transport way. first and secondcompensation means for said first and second surface position signals,respectively, to compensate for inhomogeneities in corresponding firstand second surfaces of sheets on said transport way; means comparingsaid compensated first and second surface position surfaces to obtain asheet thickness value; and means comparing said sheet thickness valuewith a predetermined value to detect superimposed sheets on saidtransport way.